If it's only for repairs, look for the old home machines that are capable of both zig-zag and straight patterns, have a step length adjustment and have a solid walking foot. But if you're planning to do some serious work - you'd better off with an industrial-grade ones, though as far as I know, older ones are only capable of one single pattern. At least last time I looked for a used one - I couldn't find anything suited.

For the few rips I used a cheapo walmart machine for the job - SQ-9000. Yes, it is a small machine and you need to smartly fold your kite. But it has all the stitches you need. I did a couple of tests with some material I had as a leftover. Things I learned: Tape your rips, hold the piece stretched and take your time.

Some of the repairs a professional repair shop could have done for 50 to 100 bucks. So depending how often you shred your kite it may not worth it the investment. I just wanted to learn something and my g/f had the machine anyway ...